1. Field of the Invention
The present invention relates to a fault diagnosis system which permits easy fault diagnosis of various automotive electronic devices.
2. Description of the Prior Art
A fault diagnosis system has been known in which data is transmitted from an electronic device on an automobile to a fault detecting means such as a tester. This type of diagnosis system has been proposed by Owatonna Tool Company, U.S.A. In this diagnosis system, the speed of transmission of data from the electronic device to the tester is determined beforehand, and the transfer of the data is conducted in response to a reference signal which is sent from the tester to the electronic device.
The basic construction of this known fault diagnosis system is shown in a block diagram in FIG. 1. An electronic device 1 and a tester 3 are connected to each other through a fault diagnosis connector 2. The electronic device 1 and the tester 3 are equipped with microcomputers 1a and 3b, respectively. The fault diagnosis connector 2 is connected both to the electronic device 1 and the tester 3, through various lines which include a transmission reference clock line 4 for a transmission reference clock signal, data lines 5, 6 for transmitting data such as items to be checked, and a transmission reference line 7 connected to a transmission reference point (GND). The transmission reference clock signal is produced in synchronization with an oscillation signal from an oscillator 3a. The portions of the data lines 5 and 6 in the electronic device 1 and in the tester 3 are provided with rectifier elements 1d and 3d, respectively, in order to prevent reversing of currents carrying the data.
The operation of the fault diagnosis system shown in FIG. 1 is as follows. In order to check the electronic device 1, a transmission reference clock signal is formed in the tester 3 in synchronization with an oscillation output from the oscillator 3a in the tester 3, and the thus formed transmission reference clock signal is transmitted through the transmission reference clock line 4 to the electronic device 1 to be checked. In consequence, the speed of data transmission from the electronic device is fixed to the same speed as the operation of the tester 3. In other words, the transmission of data from the electronic device 1 is synchronized with the operation of the tester 3, thus enabling the tester to conduct a fault diagnosis on the electronic device 1. This known fault diagnosis system is advantageous in that as long as electronic device to be checked has a serial transmission function as hardware thereof, there need not be any internal means for generating a reference clock.
Unfortunately, however, this diagnosis system cannot be applied to electronic devices which are not equipped with a serial transmission function. Therefore, when this apparatus is used in fault diagnosis of electronic devices having no serial transmission function, it is often necessary to carry out the serial transmission by a suitable port control by means of software. In such a case, the time of execution of the main program is used as the reference speed for the data transmission.
Another problem of this known fault diagnosis system is that the transmission reference clock line 4 is required in addition to the data lines 5 and 6, with the result that the adaptability of the diagnosis system to a variety of electronic devices is limited.